A disk storage apparatus, for example, a hard disk drive (HDD), conventionally carries out, besides processing of commands for read/write operations from a host, servo processing in which positioning control is performed to place a head at a target position on a disk.
A controller for the HDD generates a servo interrupt request at a sampling point of time when servo information on the disk is to be read, and shifts to servo processing in accordance with execution of servo interrupt processing. In this case, in order to preferentially carry out command processing, the controller determines not to carry out the servo interrupt processing and skips the servo interrupt processing if the command processing is being carried out. Thus, the servo interrupt may be omitted while the command processing is in execution.
The HDD carries out servo processing by a method based on a feedback control system including a state observer. In this case, the state observer carries out arithmetic processing on an internal model corresponding to a control target, and in actuality, estimates the current position of the head. The state observer carries out the model arithmetic processing so that an internal model state follows the actual head position calculated using the servo information.
Here, when the execution of the servo interrupt processing is skipped while the command processing is being carried out, no servo information is obtained during this period, precluding the actual head position from being detected. Thus, when servo processing is started immediately after the command processing ends, the state observer carries out the model arithmetic processing based on an estimated position deviating from the actual head position. Hence, in particular, an increased number of skips in the execution of the servo interrupt processing may cause a feedback control system to make undesirable responses, thus degrading the performance of the apparatus and further making servo control processing unstable.